rapdio/tsi721: Use PCI Express Capability accessors

Use PCI Express Capability access functions to simplify tsi721 driver.

[bhelgaas: use word (16-bit) accesses for PCI_EXP_DEVCTL, PCI_EXP_DEVCTL2]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>